Possibly other restrictions as well.
From Staff T website
Travellers transiting through Singapore
Staff travellers transiting through Singapore, please note that transferring to a flight on a standby ticket is currently not permitted.
Travellers transferring to a flight on a confirmed ticket on an approved carrier (i.e a carrier operating a transfer desk), must hold a confirmed onward ticket to the final destination.
Travellers holding a confirmed onward ticket must not clear immigration but proceed to the transfer desk to check in for the onward confirmed flight (and have their checked baggage transferred to their onward confirmed flight).
If a traveller clears immigration in Singapore, they must quarantine in a local hotel until a negative post arrival Covid test is received. The test result can take up to 2 days.
On receipt of a negative test result, staff travellers can then return to the Airport to travel beyond Singapore.